Link to ami angelwings on Supergirl's book
being a bit guest-star heavy. Yeah, I understated it. o_O
The sad thing is, I don't think it's just Supergirl. There seems to be a general desire among some writers to use as many trademark characters as they can in various roles instead of developing supporting casts with characters more like real people. It's a sign of overenthusiastic amateurism, like starting your comic-book writing career on a team book, Meltzer.
Wonder Woman has Nemesis & Sarge Steel now. Who needs Mike Schorr? The Question's girlfriend is Batwoman, Catwoman dated Slam Bradley, Huntress dated the previous Question... Argh.
And we have so many books built around the idea of slamming these characters into teams, sometimes for what amount to fanfic purposes: Birds of Prey, JLA, JSA, Teen Titans, Two Avengers titles, Heroes for Hire, the Initiative, on and on. Not to mention Countdown & its ilk.
Oh, well, it keeps the trademarks in print so they don't lapse, right? Whatever. It's a silly newbie writer thing, really.